用C语言写一个有头单向链表存放员工（name age gender），可以实现外部增删查改。

以下是使用C语言实现有头单向链表存放员工信息的代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// 定义员工结构体
typedef struct Employee {
    char name[20];
    int age;
    char gender[10];
    struct Employee *next;
} Employee;

// 全局变量，指向头结点
Employee *head = NULL;

// 函数声明
void addEmployee();
void deleteEmployee();
void modifyEmployee();
void searchEmployee();
void printAllEmployees();

int main() {
    int choice;
    do {
        printf("\n1. Add employee\n");
        printf("2. Delete employee\n");
        printf("3. Modify employee\n");
        printf("4. Search employee\n");
        printf("5. Print all employees\n");
        printf("6. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);

        switch (choice) {
            case 1:
                addEmployee();
                break;
            case 2:
                deleteEmployee();
                break;
            case 3:
                modifyEmployee();
                break;
            case 4:
                searchEmployee();
                break;
            case 5:
                printAllEmployees();
                break;
            case 6:
                printf("Exiting program...\n");
                break;
            default:
                printf("Invalid choice. Try again.\n");
        }
    } while (choice != 6);

    return 0;
}

void addEmployee() {
    Employee *newEmployee = (Employee *) malloc(sizeof(Employee));

    printf("Enter employee name: ");
    scanf("%s", newEmployee->name);

    printf("Enter employee age: ");
    scanf("%d", &newEmployee->age);

    printf("Enter employee gender: ");
    scanf("%s", newEmployee->gender);

    newEmployee->next = NULL;

    if (head == NULL) { // 链表为空，新节点为第一个节点
        head = newEmployee;
    } else { // 链表不为空，将新节点插入到链表头部
        newEmployee->next = head;
        head = newEmployee;
    }

    printf("Employee added successfully.\n");
}

void deleteEmployee() {
    if (head == NULL) { // 链表为空
        printf("No employees to delete.\n");
        return;
    }

    char name[20];
    printf("Enter the name of the employee to delete: ");
    scanf("%s", name);

    Employee *currentEmployee = head;
    Employee *previousEmployee = NULL;
    int found = 0;

    while (currentEmployee != NULL) {
        if (strcmp(currentEmployee->name, name) == 0) { // 找到要删除的节点
            found = 1;
            break;
        }

        previousEmployee = currentEmployee;
        currentEmployee = currentEmployee->next;
    }

    if (!found) { // 没有找到要删除的节点
        printf("Employee not found.\n");
    } else {
        if (previousEmployee != NULL) { // 要删除的节点不是头结点
            previousEmployee->next = currentEmployee->next;
        } else { // 要删除的节点是头结点
            head = currentEmployee->next;
        }

        free(currentEmployee);
        printf("Employee deleted successfully.\n");
    }
}

void modifyEmployee() {
    if (head == NULL) { // 链表为空
        printf("No employees to modify.\n");
        return;
    }

    char name[20];
    printf("Enter the name of the employee to modify: ");
    scanf("%s", name);

    Employee *currentEmployee = head;
    int found = 0;

    while (currentEmployee != NULL) {
        if (strcmp(currentEmployee->name, name) == 0) { // 找到要修改的节点
            found = 1;
            break;
        }

        currentEmployee = currentEmployee->next;
    }

    if (!found) { // 没有找到要修改的节点
        printf("Employee not found.\n");
    } else {
        printf("Enter new age: ");
        scanf("%d", &currentEmployee->age);

        printf("Enter new gender: ");
        scanf("%s", currentEmployee->gender);

        printf("Employee modified successfully.\n");
    }
}

void searchEmployee() {
    if (head == NULL) { // 链表为空
        printf("No employees to search.\n");
        return;
    }

    char name[20];
    printf("Enter the name of the employee to search: ");
    scanf("%s", name);

    Employee *currentEmployee = head;
    int found = 0;

    while (currentEmployee != NULL) {
        if (strcmp(currentEmployee->name, name) == 0) { // 找到要查找的节点
            found = 1;
            break;
        }

        currentEmployee = currentEmployee->next;
    }

    if (!found) { // 没有找到要查找的节点
        printf("Employee not found.\n");
    } else {
        printf("Name: %s\n", currentEmployee->name);
        printf("Age: %d\n", currentEmployee->age);
        printf("Gender: %s\n", currentEmployee->gender);
    }
}

void printAllEmployees() {
    if (head == NULL) { // 链表为空
        printf("No employees to print.\n");
        return;
    }

    Employee *currentEmployee = head;

    while (currentEmployee != NULL) {
        printf("Name: %s\n", currentEmployee->name);
        printf("Age: %d\n", currentEmployee->age);
        printf("Gender: %s\n", currentEmployee->gender);
        printf("\n");

        currentEmployee = currentEmployee->next;
    }
}

在该程序中，我们定义了一个员工结构体，包含员工的姓名、年龄和性别，并且还有一个指向下一个节点的指针。我们使用全局变量 `head` 来指向链表的头结点。

程序提供了以下操作：

- 添加员工信息
- 删除员工信息
- 修改员工信息
- 查找员工信息
- 打印所有员工信息

在添加员工信息时，我们先创建一个新的节点，然后从用户获取员工信息，并将新节点插入到链表头部。在删除员工信息时，我们先从用户获取要删除的员工姓名，然后遍历链表，找到对应的节点并删除。在修改员工信息时，我们要先从用户获取要修改的员工姓名，然后遍历链表，找到对应的节点并修改。在查找员工信息时，我们要先从用户获取要查找的员工姓名，然后遍历链表，找到对应的节点并输出。最后，在打印所有员工信息时，我们只需遍历链表并输出每个节点的信息即可。

我们可以根据需要对该程序进行修改，例如增加员工编号等信息。